Fastener

ABSTRACT

A shearable fastener has a threaded shank (10) adapted for engagement with a threaded bore and a head (12) formed for engagement with a drive tool. The shank (10) is formed with a series of axially separated weakenings (14) defining a series of shear planes, the weakenings (14) being formed such that the applied torque necessary to cause the shank (10) to shear increases progressively from the shear plane (14) furthest from the head (12) to the shear plane (14) nearest the head (12).

BACKGROUND OF THE INVENTION

This invention relates to improvements in fasteners, in particular toimprovements in shearable fasteners. Such fasteners are particularlysuitable for securing electrical conductors to electrical connectors.

Electrical connectors are widely used for the jointing of two or moreelectrical cables or conductors. One form of such connector comprises abody with blind bores into which the ends of the conductors areinserted. Each conductor is fixed in place by threaded fasteners whichare engaged with threaded bores in the wall of the blind bore and whichhold the conductor against the opposite internal face of the bore.Commonly, Such fasteners have shearable heads which break off at acertain predetermined applied torque. This prevents excessive andpossibly damaging, force being applied to the conductor and also resultsin the assembled joint having a reasonably smooth profile.

Known systems of the type described above suffer from the disadvantage,however that because the conductors joined together using the connectormay take various different sizes it is necessary for the shearablefasteners to be produced in a corresponding range of lengths in order toensure that the assembled joint has the desired smooth profile. Thismeans that a fitter must carry a suitable supply of all different sizesand can result in mistakes being made and inappropriate fasteners beingused.

British Patent Application No GB 2281599 describes a threaded fastenerwhich, when applied using a special tool, is arranged to shear at anaxial position along its length corresponding to the surface of anelectrical connector with which it is engaged. However, such a fastenercan only be used in conjunction with such a special tool.

There has now been devised an improved fastener, of particular use insecuring an electrical conductor to an electrical connector, whichovercomes or substantially mitigates the above mentioned disadvantage.

SUMMARY OF THE INVENTION

According, to the invention, there is provided a shearable fastenerhaving a threaded shank adapted for engagement with a threaded bore anda head portion formed for engagement with a drive tool, the shank beingformed with a series of axially separated weakenings defining a seriesof shear planes, wherein the weakenings are formed such that the appliedtorque necessary to cause the shank to shear increases progressivelyfrom the shear plane furthest from the head portion to the shear planenearest the head portion.

The fastener according to the invention is advantageous primarily inthat as the fastener is engaged with the threaded bore the shear planesenter the threaded bore in succession. As the thread on either side ofeach shear plane on the shank engages with the threaded bore, that shearplane becomes supported and this increases the torque required to shearthe fastener at that point. When rotation of the fastener is prevented,the fastener will shear at either the shear plane immediately externalto the threaded bore or at a partially supported shear plane within butimmediately adjacent to the external end of the threaded bore, dependingon which requires the lower shear torque. In either case, the fastenerwill shear substantially flush with the external end of the threadedbore with a minimal length of shank standing proud of the threaded bore.Thus, the same fastener can be used for securing conductors of differingdiameter. No special tools are required to ensure that the shank of thefastener shears at the desired shear plane. The drive tool may, forexample, be a conventional spanner or socket wrench, since the drivetool plays no part in determining the plane in which the shank of thefastener shears.

According to a further aspect of the invention, there is provided anelectrical connector comprising first and second connector parts betweenwhich an electrical conductor may be received, and a threaded fastenerengageable with a threaded bore in the first connector part such thatthe fastener may engage the conductor, either directly or throughintermediate components, and secure the conductor against the secondconnector part, the fastener having a threaded shank adapted forengagement with the threaded bore and a head portion formed forengagement with a drive tool, the shank being formed with a series ofaxially separated weakenings defining a series of shear planes, whereinthe weakenings are formed such that the applied torque necessary tocause the shank to shear increases progressively from the shear planefurthest from the head portion to the shear plane nearest the headportion. The first connector part and the second connector part may beseparate components held in fixed relation, or may be formed integrally,eg as opposite sides of a tubular socket into which the conductor isinserted.

The fastener may bear directly on the conductor. Alternatively, thefastener may drive one or more intermediate components, eg a pressureplate or an insulation piercing member, into engagement with theconductor.

The connector may contain a plurality of sockets for the connectiontogether of a corresponding number of conductors (eg two or threeconductors). Alternatively, the connector may have only a single socketand be for the connection of a single conductor to some other component.In a further alternative, a socket may receive more than one conductor.

Each conductor may be secured by a single fastener or by more than one,eg two, fasteners. Where there is more than one fastener, the threadedbores for the fasteners may be parallel and adjacent, or may have someother arrangement. For example, where the first and second connectorpart are opposite sides of a socket, each side may have one or morethreaded bores for fasteners which secure the conductor to the oppositeside of the socket.

The external surface of the connector may be curved or flat, but ineither case the threaded bore is preferably countersunk, such thatshearing preferentially takes place at a point below the outer surfaceof the connector. The spacing of the weakenings on the fastener shank ispreferably such that at least one weakening is positioned within thecountersink whenever the fastener engages a conductor.

In principle, any form of drive means may be used to rotate thefastener, but preferably the head portion of the fastener may have ahead of non-circular form, eg a square or hexagonal head, such thatpositive drive may be transmitted to the fastener using a suitable toolsuch as a socket wrench. Alternatively, the head portion of the fastenermay be formed with a bore or recess, eg of hexagonal or square section,with which an Allen key or the like may be engaged. In such a case thebore or recess need not extend far along the axis of the fastener.However, if the bore or recess does extend a substantial distance alongthe axis of the fastener, the drive tool should not extend into therecess or bore so far that it supports internally any of the weakeningswhich define the shear planes.

In a further alternative, the means by which the drive tool isengageable with the fastener may comprise one or more transverse boresor recesses. In such a case, the head portion of the fastener may simplybe a terminal region of the shank.

The weakenings in the fastener shank preferably comprise holes, groovesor slits formed in the side of the fastener. The progressive variationin the applied torque necessary to cause shearing can be achieved bycontrol of the dimensions (width, depth) of the holes, grooves or slitsconstituting the weakenings.

In a particularly preferred embodiment, the weakenings in the shankcomprise a series of axially separated annular grooves. The depth of thegrooves decreases from that nearest the tip of the shank to that nearestthe head portion. In this arrangement, in the successive shear planesdefined by the grooves, the effective diameter of the shank increasesprogressively from the tip of the shank to the head portion. Hence, theapplied torque necessary to cause shearing increases from the tip of theshank to the head portion.

Because smaller diameter conductors are less robust than large diameterconductors, it may be desirable for the clamping force exerted by thefastener on a smaller conductor to be less than in the case of a largerconductor. Preferably, therefore, the fastener is formed such that thefrictional back loading increases the further the fastener is engagedwith the threaded bore. This can be achieved, for example, by taperingthe shank such that there is a slight increase in diameter of the shankfrom the tip to the head. Alternatively, the depth of the thread may beprogressively slightly reduced as the thread approaches the head. Inother embodiments the thread may be imperfectly formed in the regionclose to the head so that the friction increases as the shank is furtherengaged with the threaded bore.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail, by way ofillustration only, with reference to the accompanying drawings, in which

FIG. 1 is a perspective view of a first electrical connector for theend-to-end connection of two electrical conductors;

FIG. 2 is a sectional view of a portion of the connector of FIG. 1;

FIG. 3 shows the same portion of the connector of FIG. 1, with ashearable threaded fastener according to the invention engaged with theconnector;

FIG. 4 is a view similar to FIG. 3, after sufficient rotations of theshearable fastener for it to engage a conductor within the connector;

FIG. 5 illustrates shearing of the shearable fastener followingcontinued application of torque to the fastener;

FIG. 6 shows the sheared fastener holding the conductor in position; and

FIG. 7 is a perspective view of a second form of electrical connectorfor the end-to-end connection of two electrical conductors.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, an electrical connector comprises a generallycylindrical aluminium body 1. Blind bores extend from each end of thebody 1 towards the middle, each bore receiving the end of respectivefirst and second conductors 2,3. Four threaded bores 4a-d are providedin the wall of the body 1. The threaded bores 4a-d receive shearablethreaded fasteners (not shown in FIG. 1) which engage the conductors 2,3and hold them in position within the blind bores in the body 1.Fasteners passing through the threaded bores 4a,4b engage the firstconductor 2 and fasteners passing through the threaded bores 4c,4dengage the second conductor 3. The connector thus far described isentirely conventional.

FIG. 2 shows a sectional view of the connector body 1 in the region ofthe threaded bore 4a.

As can be seen, the upper portion of the bore 4a is provided with acountersink 5.

In use, shearable threaded fasteners 8 (see FIG. 3) are engaged with thethreaded bore 4a, and with the other three threaded bores 4b-d. As shownin FIG. 3, the fastener 8 has a generally cylindrical shank 10 with atip 11 formed into an obtuse point. The fastener 8 has a head 12 ofnon-circular form, preferably a hexagonal cross section 18 or squarecross-section 19. The cylindrical outer surface of the shank 10 isformed with a screw thread 13 and, superimposed on the thread, a seriesof spaced apart circumferential slits 14 which extend radially into theshank 10. The slits 14 are arranged and disposed to cause shearing ofthe shank 10 upon application of certain predetermined torques. Thedepth of the slits 14 increases progressively from the head 12 of thefastener 8 towards the tip 11, such that the torque necessary to causeshearing of the shank 10 increases with distance from the tip 11.

The fastener 8 is engaged with the threaded bore 4a using a conventionaldrive tool such as a socket wrench which engages the head 12.Alternatively, the head 12 of the fastener may be formed with a bore orrecess 16, e.g. a hexagonal or square section, with which an Allen keyor the like may be engaged. For clarity, the drive tool is not shown inthe drawings. The fastener 8 is rotated until its tip 11 engages theconductor 2 (see FIGS. 3 and 4).

Continued application of torque to the fastener 8 causes the fastener 8to shear, as shown in FIG. 5. Because the lower portions of the fastener8 are supported externally by the walls of the threaded bore 4a, andbecause the applied torque necessary to cause shearing increases withdistance from the tip 11, the shank 10 shears preferentially at thefirst slit 14 which is external to the threaded bore 4a. This slit willgenerally be in the countersink 5 of the bore 4a. Occasionally, however,the fastener may shear at a slit 14 located just internal to thethreaded bore 4a. After shearing of the fastener 8, the remaining lowerportion of the fastener 8 holds the conductor 2 securely in place (seeFIG. 6).

In the case of a conductor of larger diameter than that shown in thedrawings, the fastener 8 would extend a shorter distance into theconnector bore before engaging the conductor. In this case shearingwould take place at a slit 14 nearer the tip 11 of the fastener 8.Because smaller conductors are less robust than larger conductors, itmay be desirable for a lesser securing force to be applied to such aconductor. For this reason, the depth of the thread 13 is reducedslightly towards the head 12. This increases the frictional back-loadingas the fastener 8 is progressively engaged with the threaded bore 4a,resulting in a reduced clamping force the further the shank 10 isengaged with the threaded bore 4a.

FIG. 7 shows a second form of electrical connector with which shearablebolts according to the invention may be used. This connector is intendedprimarily for low-voltage applications and comprises a connector body 21of channel cross-section. The upper part of the body 21 is closed by apair of closure members 22,23, the sides of which have retainingformations 24 which engage corresponding grooves in the body 21. Each ofthe closure members 22,23 has a pair of threaded bores 25 with whichshearable threaded fasteners such as those described above may beengaged. The connector is used in a similar fashion to the firstconnector described above.

In FIGS. 3 to 6, the differences in the dimensions of the slits 14 isexaggerated for clarity. In practice, the differences in the depth ofthe slits 14 may be only slight, and may not be apparent to the nakedeye. In one practical embodiment, the fastener 8 is of brass and has ashank 10 of nominal diameter 18 mm. The fastener 8 has an overall lengthof 41 mm, the lowermost slit 14 being 12.25 mm from the tip 11 of thefastener 8. The slits 14 are separated axially at 3.75 mm centres andare all 0.75 mm wide.

The slit 14 closest to the tip 11 has a depth of approximately 5 mm suchthat in the plane defined by that slit the shank 10 has an effectivediameter of approximately 8 mm. The shank 10 is found to shear in thatplane at an applied torque of approximately 26-30 ft.lbs. The foursucceeding slits 14 have depths which are successively reduced by 0.05mm (ie the effective diameter of the shank 10 increases by 0.1 mm fromone shear plane to the next). This leads to an increase in the torque atwhich the shank 10 will shear by approximately 2 ft.lbs from one shearplane to the next.

I claim:
 1. A shearable fastener comprising: a threaded shank adapted for engagement with a threaded bore; a head portion formed for engagement with a drive tool; and means for defining a series of shear planes on the shank, said means for defining a series of shear planes including a series of axially separated weakenings; wherein the weakenings are formed such that the applied torque necessary to cause the shank to shear increases progressively from the shear plane furthest from the head portion to the shear plane nearest the head portion when said fastener is engaged with said threaded bore.
 2. A fastener as claimed in claim 1, wherein the weakenings comprise holes, grooves or slits formed in the shank.
 3. A fastener as claimed in claim 1, wherein the weakenings comprise annular grooves, the depth of the grooves decreasing from that furthest from the head portion to that nearest the head portion.
 4. A fastener as claim in claim 1, having a configuration such that the frictional back-loading between the shank and a threaded bore with which it is engaged increases the further the shank is engaged with the threaded bore.
 5. A fastener as claimed in claim 4, wherein the diameter of the shank increases from the tip of the shank to the head portion.
 6. A fastener as claimed in claim 4, wherein the shank is formed with a thread, the depth of the thread being progressively reduced as the thread approaches the head portion.
 7. A fastener as claimed in claim 1, wherein the head portion is of non-circular cross-section.
 8. A fastener as claimed in claim 7, wherein the head portion is one of a square and hexagonal cross-section.
 9. A fastener as claimed in claim 1, wherein the head portion is formed with a recess of non-circular cross-section.
 10. An electrical connector comprising first and second connector parts between which an electrical conductor may be received, and a threaded fastener engageable with a threaded bore in the first connector part such that the fastener may engage the conductor, either directly or through intermediate components, and secure the conductor against the second conductor part, the fastener being as claimed in claim
 1. 11. A connector as claimed in claim 10, wherein a plurality of threaded bores are provided for a corresponding number of threaded fasteners to engage the conductor.
 12. A connector as claim in claim 10, wherein the first connector part and the second connector part are separate components held in fixed relation.
 13. A connector as claimed in claim 10, wherein the first connector part and the second connector part are formed integrally.
 14. A connector as claimed in claim 10, wherein at least one fastener bears directly on the conductor.
 15. A connector as claimed in claim 10, wherein at least one fastener drives one or more intermediate components into engagement with the conductor.
 16. A connector as claimed in claim 10, which comprises a plurality of sockets for the connection of a corresponding number of conductors.
 17. A connector as claimed in claim 10, which comprises a single socket.
 18. A connector as claimed in claim 10, wherein at least one threaded bore is countersunk.
 19. A fastener as claimed in claim 1, wherein the shank has a solid cross-section. 